A light emitting diode (“LED”) is a semiconductor light source. LEDs provide numerous advantages over other light sources such as incandescent lights. Amongst other advantages, LEDs typically have longer lifetimes, greater reliability, faster switching characteristics and lower energy consumption. Recent advances have produced LEDs with luminous efficiencies (lumen per Watt) that are comparable with or surpass fluorescent lights.
LEDs produce light by the spontaneous recombination of electron and hole pairs when the LED is forward biased above the threshold voltage in an electronic circuit. The wavelength of the light produced depends upon the band gap between the materials used in the p-n junction that form the LED. The wavelength of the light produced by an LED is typically in the infra red, visible or UV ranges. Detailed information on LEDs is found in “Light emitting diodes” by E. Fred Schubert, Cambridge University Press, which is hereby incorporated in its entirety by reference. Detailed information on semiconductor optics is found in “Semiconductor optics” by Claus F. Klingshirn, Springer press, which is hereby incorporated in its entirety by reference.
For ease of manufacturing, the most common form of LED is typically on the order of a micron-sized planar square LED die disposed on a substrate. The semiconductor comprising the die is usually gallium nitride, and the substrate may be a metal such as aluminum, which also functions as a heat sink. The LED die is electrically connected to circuitry on the substrate by fine metal wires. The LED die itself may be surface mounted on the substrate, or within a cavity on the substrate.
Various challenges exist in producing a LED with a high luminous output that is suitable for use as a light source in human environments. The first is maximizing light extraction from the planar LED die itself. As semiconductor materials have a high refractive index, a large quantity of the light produced undergoes total internal reflection (TIR) at the semiconductor-air interface. It is known in the art to reduce the amount of light that undergoes TIR by reducing the difference between the refractive indices at the semiconductor surface. As the semiconductor refractive index is a material characteristic, this is achieved by encapsulating the LED with an encapsulating material having a higher refractive index. Historically an epoxy material was used, and more recently silicone due to its comparatively higher transparency, color stability and thermal performance.
A single unencapsulated LED produces monochromatic light. Due to the interest in using LEDs as ambient lighting sources, in recent years research has focused on producing LED packages that emit light of different colors to that emitted by the LED die. There has been considerable interest in producing white light. The most popular way of producing white light from a single LED is by disposing a wavelength converting material, such as a yellow phosphor, on the visible (emitting) side of a blue-light emitting LED die. A layer of wavelength converting material applied on the LED die will absorb some of the emitted photons, and down-convert them into visible wavelength light, resulting in a dichromatic light source of blue and yellow wavelength light. If the yellow and blue light is produced in the correct proportions it is perceived by the human eye as a white color.
It is known in the art to add a wavelength converting material to the encapsulating layer surrounding the LED die as an alternative to depositing directly on the die. Application of the encapsulating material may be by different methods. Some methods use molding or pre-molding to fix encapsulant directly to the substrate, and some methods create and then fill a dam that encircles the LED die. The latter is usually referred to as dispensing as the encapsulating material is provided in liquid form followed by curing. Actually in this drop dispense method, a phosphor ink can also be applied to the surface of the LED die.
Another method known in the art for introducing a phosphor layer to an LED die is by using an ink-jet printer to ink-jet a phosphor ink on the surface of the LED die. This ink-jet printing method is described in U.S. patent application Ser. No. 13/103,117, the disclosure of which is incorporated by reference herein.
However, in conventional fabrication methods, phosphor particles on the order of 5-25 micrometers are typically used. Such particles have various disadvantages including settling in the dispersant, resulting in an uneven distribution of phosphors and non-uniform color emitted from the packaged device.
Thus there is a need in the art for improved phosphor particles for both drop dispense and ink-jet printing methods to form LED devices.